reflecting) adjacent wavelengths over extended spectral ranges. The wavelength
bandwidth often is described in terms of thefull-width half-maximum(FWHM)
response of afilter. As shown in Fig.5.11, the FWHM is the width of the spectral
transmission window at the half-power point.
A basic application of a dichroicfilter (or a TFF) is to separate a beam of light
into two beams with differing spectral regions, as Fig.5.12illustrates. Reciprocally,
by reversing the arrows in Fig.5.12the dichroicfilter can combine two optical
beams from different spectral bands into a single optical beam. Such a device
Av er ag e
Center transmission
wavelength
50%
Cut-off
50%
Cut-on
Longpass filter Bandpass filter Shortpass filter
Transmission (%)
Relative wavelength region
0
100
(a) (b) (c)
50
Average
Center transmission
wavelength
50%
Cut-off
50%
Cut-on
Fig. 5.11 Three categories of dichroicfilters and some terminology
450
450
450
Antireflection
coating
Dichroic
filter coating
Substrate
Incident beam
Reflected
beam
Transmitted
beam
Fig. 5.12 Characteristics of a
generic dichroic mirror for
beam splitting
5.6 Optical Filters 139